In a Long Term Evolution (LTE) system, a User Equipment (UE) has to perform blind detection for a Physical Downlink Control Channel (PDCCH) simultaneously in a common search space and a UE-specific search space, and an aggregation level of Control Channel Elements (CCEs) for the PDCCH may be 1, 2, 4 or 8. The number of PDCCH candidates to be detected blindly may vary from one aggregation level of CCEs from another, and reference is made to Table 1 depicting the description, given in the 3rd Generation Partnership Project (3GPP) TS36.213 Section 9.1.1, of PDCCH candidates monitored by the UE.
TABLE 1Search space Sk(L)Number of PDCCHTypeAggregation level LSize [in CCEs]candidates M(L)UE-specific16621264828162Common41648162
Furthermore, blind detection has to be performed for two formats of Downlink Control Information (DCI) for each transmission mode, and the different DCI formats are distinguished with their lengths. A largest number 44 of blind detections including a number 12 of ones in the common search space and a number 32 of ones in the UE-specific search space are performed at the user equipment for a DCI format at a different aggregation level. In an R8 system, the length of bits in a DCI format varies with a different bandwidth of the system, and the UE knows the size of the bandwidth of the system while performing blind detection for the DCI format, so that the number of blind detections will not be increased due to the different bandwidth of the system. Moreover in the R8 system, the UE has to perform blind detection for different DCI formats in different transmission modes, which has been defined particularly in the protocol 36.213. A specific specification is as follows taking a Cell Radio Network Temporary Identity (C-RNTI) as an example.
For a Long Term Evolution-Advanced (LTE-A) system, resources of a plurality of LTE carriers (also referred to as component carriers) have to be linked in use so as to support a wider system bandwidth, e.g., 100 MHz, etc., than the LTE system, and particularly a plurality of consecutive LTE carriers are aggregated to provide the LTE-A system with a larger transmission bandwidth or a plurality of inconsecutive LTE carriers are aggregated to provide the LTE-A system with a larger transmission bandwidth.
A study of the standardization organization exhibits such an ongoing trend that it has been commonly recognized to design a system with carrier aggregation by making a design over each carrier as consistent with the LTE R8 as possible to thereby ensure that the LTE R8 user equipment can operate normally over each component carrier.
As currently discussed about the standard of the LTE-A, there are the following two candidate schemes for a physical downlink control channel design of a system with carrier aggregation taking into account the complexity of the PDCCH design, the flexibility of scheduling and the condition of asymmetric carrier aggregation in the uplink and the downlink.
In a first scheme as illustrated in FIG. 1, a PDCCH is transmitted separately over each carrier, and a physical resource of only the present carrier can be scheduled over the PDCCH, which is also referred to as intra-carrier scheduling.
In a second scheme as illustrated in FIG. 2, a plurality of carrier resources are scheduled over a plurality of separate PDCCHs borne over one or more carriers, and this scheme is an improvement to the first scheme. In this case, a resource of only one carrier can be scheduled over each PDCCH, and for example, a plurality of PDCCHs are borne over one carrier, that is, a plurality of carrier resources are scheduled over a plurality of separate PDCCHs borne over a specific carrier, which is also referred to as across-carrier scheduling.
In the second scheme, it has been accepted in the existing standard to add a 1 to 3-bit Carrier Indicator to the original R8 scheduling signalling format to indicate which component carrier has a physical resource scheduled by this scheduling signalling, and it has been accepted in the discussion about the standard to allocate a UE Downlink Component Carrier Set (UE DL CC Set) to the UE in a specific signalling so that the UE can receive downlink data over a DL CC in the set. As suggested in some proposed schemes, a PDCCH Active CC Set is further defined, and the UE has to perform blind detection for a PDCCH in this set. What is illustrated in FIG. 2 can be taken as a case of UE DL CC Set={CC1, CC2, CC3, CC4, CC5} and PDCCH Active Set={CC3}.
Based on the foregoing first scheme, the UE has to perform blind detection similar to the R8 separately over each carrier, that is, there are a number 44 of blind detections over each carrier of each sub-frame, thus resulting in a total number 44*5=220 without taking into account an increased number of blind detections due to other new characteristics of the LTE-A, and as can be apparent, the capacity of the LTE-A UE is highly required.
Based on the foregoing second scheme, if the component carriers are provided with the same bandwidth and transmission mode, the number of blind detections by the UE, when allocating only one carrier as a PDCCH bearer carrier (as illustrated in FIG. 2), can be controlled to be the same as in the case for the R8, that is, there are a number 44 of blind detections in each sub-frame without taking into account an increased number of blind detections due to other new characteristics of the LTE-A.
Based on the foregoing second scheme, if there are different transmission bandwidths for the respective component carriers, the UE has to perform a total number 44*5=220 of blind detections in a sub-frame in the case illustrated in FIG. 2 because the length of bits in the same DCI format varies with the different transmission bandwidths. Different transmission modes are possible for each component carrier, and there may be a further increased number of blind detections by the UE, thus resulting in a number up to 44*5*5=1100 of blind detections.
Based on the second scheme, there may be also a further increased number of blind detections if the UE is configured with more than one PDCCH carriers.
Therefore, based on the second scheme, the number of blind detections by the UE may be increased unacceptably if there is no restraint for scheduling the UE DL CC Set and the PDCCH Active CC Set, and this may be adverse to both the cost of implementing the UE and the scheduling in the system.
In summary, the blind detection method is adopted for a downlink control channel in the LTE system, that is, the UE decodes a downlink control channel in each sub-frame by making a number of attempts on different resource locations of the control channel, different coding rates of the control channel and different formats of control signalling until correct control signalling is decoded correctly. The R8 UE has a specified capacity of a number 44 of blind detections in a sub-frame, and this blind detection capacity depends upon the complexity and cost of implementing the user equipment.
In the LTE-A system, various transmission modes can be defined in the downlink thanks to enhanced multi-antenna and transmission technologies so that various new formats of control signalling have emerged inevitably, and various uplink transmission modes, each of which may correspond to different formats of uplink scheduling signalling, can be defined in the system also in the uplink thanks to introduction of the multi-antenna technology, a result of which may be a larger number of blind detections to be supported by the LTE-A user equipment. Furthermore, the number of blind detections by the user equipment may be further increased due to introduction of carrier aggregation and separately encoded control signalling of each carrier. Therefore, it is an issue under consideration to control the number of blind detections.